1. FIELD OF THE INVENTION:
This invention relates to safety valves used in subterranean wells, such as oil or gas wells, and specifically to valves which are actuated in response to changes in control fluid pressure in a separate control line extending from the subsurface valve location to the well surface.
2. DESCRIPTION OF THE PRIOR ART:
Subsurface safety valves are commonly used in oil or gas wells to prevent the escape of fluids from a producing formation in the event of damage at the well surface. Typically, these valves are incorporated into or attached to the fluid transmission production tubing which is inserted through the well casing and extends from the surface of the well to the producing formation. The flow of fluids through this inner tubing string must be interrupted in the event of damage to the wellhead at the surface. By positioning these valves at a location below the well surface, for example below the mud line in an offshore well, the valve can be closed to prevent the escape of produced fluids. The most common subsurface safety valves employ either a shiftable flapper or a rotatable ball valve head to open and close the fluid transmission conduit. Commonly, either a shiftable flapper or ball valve head is actuated in response to axial movement by a flow tube actuator. In conventional ball valves, downward movement of a flow tube actuator will impart rotation to a closed ball valve head to rotate a central passage through the ball into alignment with the fluid transmission conduit. In conventional flapper valves, the flow tube will engage a closed flapper to shift the flapper to the open position by rotation about the flapper hinge.
In these conventional valves, axial movement of the flow tube actuator is normally imparted by an increase in control fluid pressure. Hydraulic control fluid is generally stored in an external control line extending from the valve to the well surface. An increase in pressure can be initiated at the well surface and this increase in pressure is transmitted through the hydraulic control fluid to the valve. Conventionally, unbalanced surfaces on the flow tube actuator are exposed to control pressure and an increase in control pressure will result in an increase in the control force acting on the flow tube. Eventually, a sufficient pressure force will be created to urge the flow tube actuator downwardly relative to the closed flapper or ball valve head and against the action of the well pressure and any spring which might be used to hold the flow tube and the valve head in the closed position. When control fluid pressure is reduced or removed, be it intentionally or as a result of damage to the external control line, the well pressure and spring acting on the flow tube actuator will be sufficient to shift this actuator upwardly permitting the valve head to close.
In addition to these conventionally actuated valves, some subsurface safety valves employ shiftable spools, pistons, or pilot valve members for actuating a flow tube actuator. These separate shiftable spools, pistons, or pilot valves may be employed to reduce the surface area upon which hydraulic fluid acts. A reduction in surface area means that the hydraulic pressure force acting to actuate the valve is less, thus permitting the safety valve to be positioned at greater depths. U.S. Pat. No. 4,005,751, U.S. Pat. No. 4,119,146 and U.S. Pat. No. 4,161,219 each disclose subsurface safety valves having a spool, piston, or pilot valve mounted in the valve housing or in a sidepocket mandrel.